A difficulty arises when we come to potassium. Potassium has an atomic number of 19. For potassium and calcium, the 4s orbital dips in energy below the 3d orbitals, and so electrons go into the 4s ...

One calcium and two chlorine atoms are near each other. An electron from each atom feels the attraction from the other atom's nucleus. The attraction by the chlorine atoms is stronger than the ...